1. Field of the Invention
The present invention relates to a surveying instrument which incorporates a magnetic incremental rotary encoder suitable for surveying instruments such as total stations theodolites or the like.
2. Description of the Related Art
Some conventional surveying instruments such as total stations, theodolites or the like are provided with an incremental rotary encoder as an angle measuring device. Conventionally, an optical incremental rotary encoder is frequently used because it has a high degree of stability and precision.
Similar to an optical incremental rotary encoder, a magnetic incremental rotary encoder is also known as an angle measuring device. A magnetic incremental rotary encoder is generally provided with a magnetic drum (graduator disc) and a magnetic sensor. The magnetic incremental rotary encoder is provided, on an outer peripheral surface of the magnetic drum thereof, with a multi-pole magnetized layer having a plurality of magnetized divisions equally divided by a number of divisions p (xe2x80x9cpxe2x80x9d being a positive integer). The magnetic sensor is positioned so as to face the multi-pole magnetized layer. This magnetic sensor is provided thereon with, e.g., four magnetoresistor elements (for instance, these four magnetoresistor elements are divided into two groups: an A-phase and a B-phase which are arranged to have a phase angle of xc2xc or xe2x85x9c pitch angle therebetween) which are disposed at predetermined intervals whose pitch is smaller than that of the plurality of magnetized divisions of the multi-pole magnetized layer to detect the variation in the resistance values of the four magnetoresistor elements, which vary in accordance with the rotation of the magnetic drum to thereby determine the zero crossing points of the outputs of the A-phase and the B-phase and the rotational angle of the magnetic drum according to an interpolative calculation xe2x80x9ctanxe2x88x921 (Aout/Bout)xe2x80x9d, wherein Aout and Bout represent the outputs of the A-phase and the B-phase, respectively.
In this type of magnetic incremental rotary encoder using magnetoresistor elements, it is ideal that a curve showing a variation of the magnetic reluctance of the magnetoresistor elements be substantially symmetrical about the zero point of the magnetic field. However, in reality, the variation curve is asymmetrical about the zero point of the magnetic field due to hysteresis which occurs in the vicinity of the zero point of the magnetic field. This is due to the fact that the position at which a magnetic domain wall is generated by the multi-pole magnetized layer of the magnetic drum varies in accordance with the direction of the magnetic field generated by the multi-pole magnetized layer of the magnetic drum. If the variation curve is asymmetrical about the zero point of the magnetic field in such a manner, a large error occurs when the magnetic drum is rotated slightly by a small angle of rotation.
Further, in a magnetic incremental rotary encoder, the resistance of each magnetoresistor element varies if the encoder suffers from an external magnetic field. This causes an error in the measured value. Accordingly, in the case of a surveying instrument which incorporates a magnetic incremental rotary encoder using magnetoresistor elements, the surveying instrument has to be designed to be immune from external magnetic fields.
An object of the present invention is to provide a surveying instrument which incorporates a magnetic incremental rotary encoder which is immune from hysteresis and external magnetic fields.
To achieve the object mentioned above, according to the present invention a surveying instrument is provided which incorporates a magnetic incremental rotary encoder, including a rotary magnetic drum provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of equally-divided magnetized divisions; and at least two magnetic sensors, each of which detects a magnetic field generated by the multi-pole magnetized layer therefrom. Each of the at least two magnetic sensors detects a variation in the magnetic field due to a rotation of the rotary magnetic drum to measure an angle of rotation of the rotary magnetic drum, and the at least two magnetic sensors are arranged so that two of the at least two magnetic sensors are offset from each other to have a phase angle of an odd-numbered pitch angle therebetween.
In an embodiment, the at least two magnetic sensors are two magnetic sensors positioned closely to each other.
In an embodiment, the at least two magnetic sensors are two magnetic sensors which are located on opposite sides of the rotary magnetic drum to be offset from each other by approximately 180 degrees about an axis of the rotary magnetic drum.
According to another aspect of the present invention, a surveying instrument is provided which incorporates a magnetic incremental rotary encoder, including a rotary magnetic drum provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of equally-divided magnetized divisions; and a magnetic sensor, which detects a magnetic field generated by the multi-pole magnetized layer therefrom, the magnetic sensor including a plurality of pairs of magnetoresistor elements. The magnetic sensor detects a variation in the magnetic field due to a rotation of the rotary magnetic drum to measure an angle of rotation of the rotary magnetic drum, and each pair of the plurality of pairs of magnetoresistor elements are arranged to be offset from each other so as to have a phase angle of an odd-numbered pitch angle therebetween.
In an embodiment, another magnetic sensor is located on the rotary magnetic drum to be offset from the other magnetic sensor about an axis of the rotary magnetic drum.
In an embodiment, the magnetic sensor has four pairs of magnetoresistor elements which are arranged so that any adjacent two pairs thereof are offset from each other so as to have a phase angle of 1/2 pitch angle therebetween.
In an embodiment, the magnetic sensor includes a first group of four pairs of magnetoresistor elements which are arranged so that any adjacent two pairs thereof are offset from each other so as to have a phase angle of 1/2 pitch angle therebetween; and a second group of four pairs of magnetoresistor elements which are arranged so that any adjacent two pairs thereof are offset from each other so as to have a phase angle of 1/2 pitch angle therebetween. The first group and second group are arranged so as to have a phase angle of 3/4 pitch angle therebetween.
Preferably, the surveying instrument is a total station.
According to another aspect of the present invention, a surveying instrument is provided, including a leveling board; a pedestal coupled to the leveling board to be rotatable about a vertical axis relative to the leveling board; a collimating telescope coupled to the pedestal to be rotatable about a horizontal axis relative to the pedestal; a horizontal-angle measuring device for measuring an angle of rotation of the pedestal relative to the leveling board; and a vertical-angle measuring device for measuring an angle of rotation of the collimating telescope relative to the pedestal. At least one of the horizontal-angle measuring device and the vertical-angle measuring device includes a magnetic incremental rotary encoder. The magnetic incremental rotary encoder includes a rotary magnetic drum provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of equally-divided magnetized divisions, a first magnetic sensor, and a second magnetic sensor; wherein the first and second magnetic sensors each include a plurality of pairs of magnetoresistor elements and are located on opposite sides of the rotary magnetic drum so that each of the first and second magnetic sensors faces the multi-pole magnetized layer. Each of the first and second magnetic sensors detects a magnetic field generated by the multi-pole magnetized layer therefrom. Each of the first and second magnetic sensors detects a variation in the magnetic field due to a rotation of the rotary magnetic drum to measure an angle of rotation of the rotary magnetic drum. Each pair of the plurality of pairs of magnetoresistor elements are arranged to be offset from each other so as to have a phase angle of an odd-numbered pitch angle therebetween.
The present disclosure relates to subject matter contained in Japanese Patent Application No.11-301117 (filed on Oct. 22, 1999) which is expressly incorporated herein by reference in its entirety.